Seismic geophysical monitoring and surveying is used in a variety of applications. For example in the oil and gas sector seismic surveys may be conducted at numerous different stages of well construction and operation. In particular, once well construction has been completed and the wells are operational there may be a desire to perform long term seismic monitoring to monitor for any microseismic events so as to highlight any significant changes in the condition of the wells and/or the reservoir over time.
Seismic monitoring may also be used for assessing reservoirs for the storage of hazardous or unwanted materials, for example in carbon dioxide sequestrations schemes. In these applications there may again be a desire to undertake long term seismic monitoring, for example to listen for microseismic events following the injections, to monitor the condition of the site over time.
Long term seismic monitoring is conventionally performed by locating an array of seismometers in an area to be monitored. Typically an array of geophones is used as the sensing array. The geophones may be arranged over the surface of the area to be monitored in a desired pattern and/or a string of geophones may be deployed down a well bore.
Recently is has been proposed to use fibre optic distributed acoustic sensors as the sensor array in seismic monitoring. Distributed acoustic sensing (DAS) is a known type of sensing where an optical fibre is deployed as a sensing fibre and interrogated with electromagnetic radiation. Radiation which is backscattered from within the optical fibre is detected and analysed to reveal information about acoustic stimuli acting on the optical fibre in different longitudinal sections of the sensing fibre, i.e. channels. Thus the DAS sensor effectively acts as a linear sensing array of sensing portions of optical fibre. The length of the sensing portions of fibre is determined by the characteristics of the interrogating radiation and the processing applied to the backscatter signals but typically sensing portions of the order of 10 m or so may be used in some applications and smaller sensing portions for more precise applications. Note as used herein the term acoustic shall mean any type of pressure wave or disturbance that may result in a change of strain on an optical fibre and for the avoidance of doubt the term acoustic be taken to include seismic waves.
DAS has several potential advantages compared to the use of geophone arrays. Firstly geophone arrays are expensive. Thus, for active surveys, where the area under investigation is stimulated using a seismic source and the response to the seismic stimulus recorded, a geophone array may be deployed just for the survey and recovered afterwards for use at another location. For long term monitoring of an area however clearly the sensor will remain deployed for the duration of the monitoring. The expense of geophone arrays means that the number of individual sensing elements in a geophone array which is deployed for long term monitoring is typically limited.
DAS however uses a relatively inexpensive optical fibre as the sensing medium. The optical fibre can be deployed in the area to be surveyed, for example by being buried in a desired arrangement to protect the fibre from the environment, and left in situ for a long period of time. With DAS a fibre of length of up to about 40 km can be used to provide surface seismic monitoring with 10 m long sensing portions to provide 4000 individual sensing portions. This provides significantly more data channels than is usual with a conventional geophone array, and at very low cost. DAS can also allow the whole of a deep well, say 4 km or more, to be monitored, possibly with much shorter sensing portions. With a typical geophone array there may be a limit to the number of geophones that are used and thus the geophone array may only be able to monitor part of a deep-well site.
For current DAS sensors the instantaneous output from any individual sensing portion of fibre may not offer the same level of sensitivity as a conventional geophone. However various processing techniques such as combining the results from several independent sensing channels can be used to improve the signal to noise ratio of the DAS sensor such that DAS can usefully be employed in seismic monitoring.
DAS therefore offers several advantages for seismic monitoring and has usefully been employed in seismic surveying. However the fact that DAS allows a significant increase in the number of sensing channels available, coupled with the fact that DAS sensor typically have a data output rate which is greater than for conventional geophones, raises some potential problems with data storage and processing.
For long term microseismic monitoring the sensor array may be acquiring data constantly for relatively long periods of time. Usually the output data from the sensor array is stored for later processing.
For seismic monitoring using a DAS sensor the significant increase in number of sensing channels, coupled with the increased data rates, will result in significantly more data being produced than with a conventional geophone array. Handling and storing this data is not a trivial task, especially as the area being monitored may often be in a remote and relatively hostile environment.
Some conventional geophone acquisition systems can be equipped with online event detection and triggering to store detected events only. In other words the data output from the geophone array may be buffered and initially processed to determine whether there are any events of interest. If not the data will not be stored and will be gradually replaced in the buffer by newer data. If however an event of interest is detected the buffered data may be stored, with data being stored until the event of interest ends. In this way only the data relevant to events of interest is stored, thus reducing data storage requirements.
With geophone arrays the relatively low data rates and relatively high sensitivity of the individual geophones means that event detection is relatively straightforward. However with DAS sensors the higher data rate and relatively lower sensitivity of an individual sensing portion of fibre means that it is not straightforward to detect when an event of interest is occurring. Processing the data from several different channels of the DAS sensor in order to detect an event of interest would involve significant computational overhead, and in order to usefully be able to make a decision whether to store or discard the buffered data the processing must operate effectively in real time. This would involve deploying significant computational resources in a field based interrogator unit with a significant impact on cost and complexity of the unit.
It would therefore be advantageous to provide methods and apparatus for seismic monitoring using distributed fibre optic sensing which mitigate at least some of the above mentioned disadvantageous.